Diesel engines produce an exhaust gas that contains a variety of pollutants that are the subject of environmental legislation around the world. These pollutants include carbon monoxide (CO), unburned hydrocarbons (HCs), oxides of nitrogen (NOx) and particulate matter (PM). To meet permissible levels of pollutants that may be emitted into the atmosphere set by legislation, exhaust systems for treating the exhaust gas produced by a diesel engine generally contain several emissions control devices. In such exhaust systems, the exhaust gas is usually passed to a first emissions control device that is able to oxidise carbon monoxide (CO) and the unburned hydrocarbons (HCs) that are present in the gas.
In addition to the pollutants mentioned above, the exhaust gas may contain impurities that originate from the specific type of fuel that has been used. Diesel fuel often comprises sulfur-containing impurities. Under the lean conditions used for combustion in a diesel engine, the sulfur-containing impurities typically form oxides of sulfur. These oxides of sulfur can poison some of the catalytically active metals in the emissions control devices of an exhaust system. In addition, the oxides of sulfur can form particulate matter or can contribute to the formation of acid rain when passed into the atmosphere.
Many countries have implemented legislation to restrict the maximum amount of sulfur that is allowed in diesel fuel. Aside from the environmental benefits, such legislation also facilitated the use of new emissions control technologies that could not tolerate the previous high levels of sulfur in diesel exhaust gas. Despite the reduction in sulfur content, the activity of some emissions control devices can still deteriorate through poisoning by oxides of sulfur.